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Achernar is the End of the River

Achernar is flattened because it rotates rapidly

Achernar – also known as Alpha Eridani – is the 9th brightest star in Earth’s sky and the flattest star known.

The 9th brightest star in all the heavens, Achernar is a well-known sight to observers in the Southern Hemisphere, but known only by name to the great majority of observers in the Northern Hemisphere. It shines brightly with a magnitude of 0.45 – in other words, as brightly as the brightest stars – but it is so far south on the dome of stars surrounding Earth that it never rises above the horizon from any location north of about 33 degrees N. latitude. Follow the links below to learn more about Achernar, sometimes called the end of the River.

In a dark sky, you can see that Achernar marks the end of a great stream of stars known to the ancients as a celestial River. This is the constellation Eridanus the River, and Achernar is its brightest star. In fact, the name Achernar means End of the River.

How to see Achernar. For all practical purposes, you must be even further south – around 25 degrees N. latitude – to see Achernar well. That is a line drawn around the entire globe passing through, say, Miami in the U.S. state of Florida and Taipei in China.

Nowhere in North America has it easy, seeing this star. For example, from Key West, Achernar rises only about 8 degrees above the southern horizon. Even farther south, from the southern tip of Hawaii’s Big Island, Achernar never quite makes it to 14 degrees.

And yet, if you are far-enough south, you can see Achernar easily. After all, this star is very bright!

Just as Eridanus marks the end of the River, so the River has a beginning, the star Beta Eridani or Cursa, which itself is easily visible from the northern hemisphere, very near Orion’s brightest star, Rigel.

On most nights of the year, Achernar cannot be seen from anywhere in North America, but around October 20 it skirts the southern horizon around midnight, never getting very high. Then as the months pass, it is seen earlier at night – around 10 p.m. in November, 8 p.m. in December and just after sunset in January. Being far to the south with no bright stars around it, Achernar stands out in its isolation. If you have a dark sky, and are far enough south, you’ll easily see Achernar’s constellation Eridanus swelling up in a great loop under the constellation Orion.

The star Achernar in the constellation Eridanus: End of the river.

Achernar’s history and mythology. The very name Achernar derives from an Arabic phrase meaning the end of the river.

Interestingly, in early classical times the name Achernar was given to the star we now know as Theta Eridani, or Acamar. At that time Acamar was the brightest star of the constellation visible from Greece, and thus was considered the River’s end.

When voyagers discovered the brighter star farther to the south, it became Achernar, and the former Achernar became Acamar.

Apparently both names derive from the same phrase, “Al Ahir al Nahr” according to Richard Hinckley Allen, and mean the same thing.

Achernar, the flattest star known

Science of Achernar. Data from the Hipparcos mission puts Achernar at about 144 light-years away. It is a B3V star, meaning that it belongs to the ranks of “normal” stars known as the “main sequence,” but it is much hotter and brighter than the sun.

In fact, it is nearly 1100 times as bright, visually, as our neighborhood star. Brighter, hotter (and bluer) than the sun, Achernar produces more energy in the non-visible ultraviolet (UV) wavelengths. When you take this into consideration, it pumps out some 3,000 to as much as 5,000 times the solar energy level. The discrepancy is due to an uncertainty in the amount of UV radiation it produces.

Achernar’s mass is 6-8 times that of our sun, and its average diameter is nearly 8-10 times that of the sun. But, while our sun spins on its axis once about every 25 days, Achernar completes one rotation in slightly more than two days, or nearly 15 times faster than our sun. This fast rotation produces an odd, flattened shape, first discovered by the European Southern Observatory’s Very Large Telescope (VLT) in 2003. Up close, Achernar would look more like a blue M&M, while our sun would look more like an orange. Read more about Achernar’s flattened shape from ESO.

This flattening of Achernar makes an exact surface temperature for this star hard to determine, because the flattening actually causes the star’s poles to be hotter than the equator. Estimates range from about 14,500 to 19,300 k (around 26,000 to about 32,000 F).